T. K. Kim

Bio: T. K. Kim is an academic researcher. The author has contributed to research in topics: Condensed matter physics & Physics. The author has an hindex of 1, co-authored 1 publications receiving 552 citations.

New Magnetic Material Having Ultrahigh Magnetic Moment

Abstract: The change of the saturation magnetization of Fe films with the pressure of nitrogen during deposition ranging from 2 × 10−5 to 7 × 10−3 Torr has been investigated systematically. We found a new magnetic material which has the highest saturation magnetization at room temperature, 2050 G, among those of all the magnetic materials. This was attributed to Fe16N2, which has a bct structure, and the magnetic moment associated with Fe atoms of Fe16N2 was deduced to be 3.0 μB.

Direct observation of the spin–orbit coupling effect in magnetic Weyl semimetal Co3Sn2S2

Abstract: Abstract The spin–orbit coupling (SOC) lifts the band degeneracy that plays a vital role in the search for different topological states, such as topological insulators (TIs) and topological semimetals (TSMs). In TSMs, the SOC can partially gap a degenerate nodal line, leading to the formation of Dirac/Weyl semimetals (DSMs/WSMs). However, such SOC-induced gap structure along the nodal line in TSMs has not yet been systematically investigated experimentally. Here, we report a direct observation of such gap structure in a magnetic WSM Co 3 Sn 2 S 2 using high-resolution angle-resolved photoemission spectroscopy. Our results not only reveal the existence and importance of the strong SOC effect in the formation of the WSM phase in Co 3 Sn 2 S 2 , but also provide insights for the understanding of its exotic physical properties.

Tuneable electron–magnon coupling of ferromagnetic surface states in PdCoO2

Abstract: Controlling spin wave excitations in magnetic materials underpins the burgeoning field of magnonics. Yet, little is known about how magnons interact with the conduction electrons of itinerant magnets, or how this interplay can be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a strong and highly-tuneable electron-magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO$_2$, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling can be enhanced 7-fold with increasing surface disorder, and concomitant charge carrier doping, becoming sufficiently strong to drive the system into a polaronic regime, accompanied by a significant quasiparticle mass enhancement. Our study thus sheds new light on electron-magnon interactions in solid-state materials, and the ways in which these can be controlled.

Pseudogap suppression by competition with superconductivity in La-based cuprates

Abstract: We have carried out a comprehensive high-resolution angle-resolved photoemission spectroscopy (ARPES) study of the pseudogap interplay with superconductivity in La-based cuprates. The three systems La 2 − x Sr x CuO 4 , La 1 . 6 − x Nd 0 . 4 Sr x CuO 4 , and La 1 . 8 − x Eu 0 . 2 Sr x CuO 4 display slightly different pseudogap critical points in the temperature versus doping phase diagram. We have studied the pseudogap evolution into the superconducting state for doping concentrations just below the critical point. In this setting, near optimal doping for superconductivity and in the presence of the weakest possible pseudogap, we uncover how the pseudogap is partially suppressed inside the superconducting state. This conclusion is based on the direct observation of a reduced pseudogap energy scale and re-emergence of spectral weight suppressed by the pseudogap. Altogether these observations suggest that the pseudogap phenomenon in La-based cuprates is in competition with superconductivity for anti-nodal spectral weight.

Superconducting dome and pseudogap endpoint in Bi2201

Abstract: Citation for published version (APA): Berben, M., Smit, S., Duffy, C., Hsu, Y-T., Bawden, L., Heringa, F., Gerritsen, F., Cassanelli, S., Feng, X., Bron, S., van Heumen, E., Huang, Y., Bertran, F., Kim, T. K., Cacho, C., Carrington, A., Golden, M. S., & Hussey, N. E. (2022). Superconducting dome and pseudogap endpoint in Bi2201. Physical Review Materials, 6(4), [044804]. https://doi.org/10.1103/PhysRevMaterials.6.044804

Perspectives on Permanent Magnetic Materials for Energy Conversion and Power Generation

Abstract: Permanent magnet development has historically been driven by the need to supply larger magnetic energy in ever smaller volumes for incorporation in an enormous variety of applications that include consumer products, transportation components, military hardware, and clean energy technologies such as wind turbine generators and hybrid vehicle regenerative motors. Since the 1960s, the so-called rare-earth “supermagnets,” composed of iron, cobalt, and rare-earth elements such as Nd, Pr, and Sm, have accounted for the majority of global sales of high-energy–product permanent magnets for advanced applications. In rare-earth magnets, the transition-metal components provide high magnetization, and the rare-earth components contribute a very large magnetocrystalline anisotropy that donates high resistance to demagnetization. However, at the end of 2009, geopolitical influences created a worldwide strategic shortage of rare-earth elements that may be addressed, among other actions, through the development of rare-earth-free magnetic materials harnessing sources of magnetic anisotropy other than that provided by the rare-earth components. Materials engineering at the micron scale, nanoscale, and Angstrom scales, accompanied by improvements in the understanding and characterization of nanoscale magnetic phenomena, is anticipated to result in new types of permanent magnetic materials with superior performance.

Advances in nanostructured permanent magnets research

Abstract: This paper reviews recent developments in research in nanostructured permanent magnets (hard magnetic materials) with emphasis on bottom-up approaches to fabrication of hard/soft nanocomposite bulk magnets. Theoretical and experimental findings on the effects of soft phase and interface conditions on interphase exchange interactions are given. Synthesis techniques for hard magnetic nanoparticles, including chemical solution methods, surfactant-assisted ball milling and other physical deposition methods are reviewed. Processing and magnetic properties of warm compacted and plastically deformed bulk magnets with nanocrystalline morphology are discussed. Prospects of producing bulk anisotropic hard/soft nanocomposite magnets are presented.

Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single‐crystal films (invited)

Abstract: Single‐crystal Fe16N2 films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density Bs of Fe16N2 films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of Bs has been measured. Bs changed with temperature reversibly up to 400 °C, while beyond 400 °C, Bs decreased irreversibly. X‐ray diffraction showed that Fe16N2 crystal is stable up to 400 °C, while beyond 400 °C, Fe16N2 dissolves into Fe and Fe4N, and also some chemical reactions between Fe16N2 and the substrate occurs. This caused the temperature dependence of Bs mentioned above. From the temperature dependence of Bs up to 400 °C, the Curie temperature of Fe16N2 is estimated to be around 540 °C by using the Langevin function. The above mentioned Bs of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic mom...

Epitaxial growth and magnetic properties of Fe16N2 films with high saturation magnetic flux density (invited)

Abstract: Fe‐N films with thicknesses of 70–1000 A were deposited by MBE onto Fe films which were epitaxially grown onto GaAs(100) substrates. Without the Fe layer, epitaxially grown Fe‐N films could not be obtained due to a reaction between Fe‐N and GaAs. TEM observations and x‐ray diffraction patterns showed that the epitaxially grown Fe‐N films consist of Fe16N2 and Fe, and that crystal orientation is Fe16N2 (001)//Fe(110). It was found that the saturation magnetic flux density (Bs) increases as the thickness of the Fe‐N films decreases. This is because the volume ratio of Fe16N2 in the Fe‐N films increases with decreasing Fe‐N film thickness. The maximum value for Bs is 2.66 T, and the volume ratio is 85%. These results indicate that Fe16N2 has a high saturation magnetic flux density of 2.8–3.0 T.